I too broke off a tap in my 302 heads. The AIR holes in my heads were about exactly 1/4". I found a 5/16" tap and found the proper drill bit, then drilled the first inch or a little more. Tapped to 5/16 - 18. I had a bunch of 1" Grade 2 bolts so I ran-em in as far as I could and cut off the heads. Messy but OK. I used a small punch to work on the broken off tap, then filled the hole, with the remainder of the tap still inside, with JB Weld. There's been no sign of leakage from any of the holes. I believe when the exhaust gases can't travel through, the holes carbon up pretty quickly.

I believe if you drilled out the holes so they were clean that JB Weld would be all you needed. In my case I had too many bolts.

R.